Dewaxing process

ABSTRACT

An improved process for the solvent dewaxing of petroleum oil stocks. The dewaxing solvent is prechilled and added incrementally or continuously at several points along the height of a cooling tower. The oil-solvent mixture is agitated during the chilling step to permit substantially instantaneous mixing of the prechilled solvent with the oil.

United States Patent 1151 Hislop et a]. Nov. 20, 1973- [54] DEWAXINGPROCESS 3,038,854 6/l962 Kiersted 208/37 [75] lnvemms HisloP; JohnEagell 21041 11 122 5/1936 522: 383/3; of Sam, Omam, Canada 2,361,503 101944 Schutte et al... 196 145 Assignee: Esso Research and Engineering "l.1 Company, Linden, NJ.

[22] Filed: 1971 Primary Examiner-Herbert Levine [21] AppL 129 73Attorney-Pearlman and Schlager Related US. Application DataContinuation-impart of Ser. No. 17,869, March 9, 1970, abandoned, whichis a continuation-in-part of Ser. No. 666,268, Sept. 8, 1967, abandoned.

US. Cl. 208/33, 196/145, 208/37,

ABSTRACT An improved process for the solvent dewaxing of pe- 1 troleumoil stocks. The dewaxing solvent is prechilled and added incrementallyor continuously at several points along the height of a cooling tower.The oilsolvent mixture is agitated during the chilling step to permitsubstantially instantaneous mixing of the prechilled solvent with theoil.

23 Claims, 1 Drawing Figure DEWAXING PROCESS CROSS-REFERENCE TO RELATEDAPPLICATIONS This is a continuation-in-part of co-pending application,Ser. No. 17,869, filed Mar. 9, I970, now abandoned which is acontinuation-in-part of Ser. No. 666,268, filed Sept. 8, I967, and nowabandoned.

BACKGROUND OF THE INVENTION This invention relates to a process fordewaxing petroleum oil stocks. More particularly, this invention relatesto a process wherein selective dewaxing solvents are used to facilitatethe precipitation of wax from waxy petroleum oil stocks. Still moreparticularly, this invention relates to a process wherein the wax ispre- .cipitated by incremental or continuous addition of a prechilledselective solvent along the height of a vertical, stirred chillingtower.

It is known in the prior art to dewax petroleum oil stocks by cooling anoil/solvent solution in ascraped surface exchanger. In this typeprocess, the oil and selective solvent are admixed in such a manner asto effect thorough solution of the oil in the solvent. The extent ofdilution is dependent upon the particular oil and the particular solventemployed and is adjusted to facilitate easy handling and optimumfiltration rates. The solution is cooled at a uniformly slow coolingrate; e.g., l8F./minute, under conditions which are controlled so as toavoid any substantial agitation of the solution during precipitation ofthe wax.

Notwithstanding the carefully controlled conditions used in this typeprocess, there are several deficiencies which hamper successfulcommercial operation. Most significant among these deficiencies is theloss of good heat transfer due to wax deposition on the exchangesurfaces. Such fouling has been repeatedly noted after short periods ofoperation; e.g., 24-48 hours. Associated directly with the loss of goodheat transfer is the loss of careful control over the cooling rate and acorresponding loss of uniform crystal growth. This nonuniform crystalgrowth results in lower filtration rates. The high pressure drop throughthe chilling section also reduces the maximum feed rate attainable.Physical mashing of the wax crystals by the action of the scrapers mayalso contribute to poor filtration.

It is also known in the prior art to dewax petroleum oil stocks bycooling in scraped surface exchangers using an incremental solventaddition technique. In this technique, the solvent is added at severalpoints along the chilling apparatus. The waxy oil is chilled withoutsolvent until some wax crystallization has occurred and the mixture hasthickened considerably. The first incre ment of solvent is introduced atthis point and cooling continues. Each incremental portion of solvent isadded as necessary to maintain fluidity until the desired separationtemperature is reached at which point the remainder of the solventdesired for filtration is added.

Using this common industrial technique, it is well known and has beenrepeatedly demonstrated that the temperature of the incrementally addedsolvent should be the same as that of the main stream at the point ofaddition. Having the solvent at a lower temperature causes shockchilling of the slurry at that point, with resulting formation ofcrystal fines, and impairment of filter rate; having the solvent warmerthrows an unnecessary additional load on the scraped surface chillers.

2 4 The bulk of the chilling of the slurry .in this well-known processis accomplished through the walls of the scraped surface chillers ratherthan by means of cold solvents.

It is also known in the art, as described in U.S. Pat. No. 2,361,503 toSchutte et al., to subject lubricating oil fractions to agitation in amultistage tower with water or brine which also serves as the coolingmedium. This process suffers from the disadvantage that the coolingmedium is completely immiscible with the wax and oil and rapidseparation occurs between the feed and water unless the mixture ismaintained in an agitated state or is emulsified. The use of water asthe cooling medium also practically limits the process to an upfiowoperation.

BRIEF SUMMARY According to this invention, waxy petroleum stocks aredewaxed bycooling the petroleum oil stock in an elongated stirredvessel, preferably a vertical tower, with a prechilled solvent that willsolubilize at least a portion of the oil stock. The solvent ispreferably introduced incrementally along the length of the vessel. Itis necessary that the degree of agitation in the stirred, elongatedvessel be sufficient to permit substantially instantaneous mixing of theoil and solvent. The prechilled solvent is added in an amount sufficientto permit easy handling and good filtration rates. The wax, whichprecipitates during the cooling stage, may be separated from thesolution by conventional means known in the art. Following separation ofthe wax, the oil, which has a reduced pour and cloud point, is recoveredfrom the solution by means well known in the art.

BRIEF DESCRIPTION OF THE DRAWING The FIGURE is a flow diagram of thedewaxing process of the invention.

DETAILED DESCRIPTION solvent for the oil stock is passed from storagetank 5 through line 6, through heat exchangers 7 and 8, where thesolvent temperature is reduced to a level sufficient to cool the oil tothe desired dewaxing temperature. Coolant enters the heat exchangers 7and 8 through lines 24 and 25, respectively, and leaves through lines 26and 27. The solvent leaves the heat exchanger 8, through line 9, andenters manifold 10. The manifold comprises a series of parallel linesproviding solvent inlets 11 to the several stagesof the cooling tower 3.The rate of flow through each inlet is regulated by flow control means(not shown), The rate of solvent flow is regulated so as to maintain adesired temperature gradient along the height of the cooling tower 3.

The first portion or ihcrement of the solvent enters the first stage,4(a), of the cooling tower 3 where it is substantially instantaneouslyadmixed with oil due to theaction of the agitator 12(a). The agitator isdriven by a variable speed motor 13 and the degree of agitation iscontrolled by variation of the motor speed, with due allowance for theflow rate through the cooling tower. The oil-solvent mixture may passupwardly or downwardly through the cooling tower 3 (downward flow onlyhas been shown). Additional prechilled solvent is introduced into atleast a portion of the several stages 4, through inlets 11 so as tomaintain a substantially constant cooling rate and at the same time toprovide the desired degree of dilution. It should be noted that anynumber of stages, for example fifty may be employed; however, it isdesirable that at least six stages be used.

The oil-solvent solution with precipitated wax passes from the finalstage of the cooling tower through line 14 to means for separating thewax from said solution 15. Any suitable means, such as filtration orcentrifugation, for such separation may be employed. The waxsolvent isremoved from the separation means through line 16. The solvent isrecovered in a suitable separating system 19, which is preferably adistillation tower,

, through line 17 and the wax exits through line 18. The

oil-solvent solution leaves the wax separation means through line 20 andpasses to means for separating the oil from solution 21. Any suitablemeans for this separation may be used, such as distillation or selectiveadsorption. The oil is removed from the separator and is recoveredthrough line 22. The solvent is removed through line 23.

Any waxy petroleum oil stock or distillate fraction thereof may bedewaxed with the process of this invention. In general, these oil stocksor distillate fractions will have a boiling range within the broad rangeof about 500F. to about 1300F. The preferred oil stocks are thelubricating oil and specialty oil fractions boiling within the range of550F. and 1200F. These fractions may come from any source, such as theparaffinic crudes obtained from Aramco, Kuwait, the Panhandle, NorthLouisiana, Tia Juana, etc.

Any selective solvent may be used in the process of this invention.Representative examples of such solvents are (a) the aliphatic ketoneshaving from 3 to 6 carbon atoms, such as acetone, methyl ethyl ketone(MEK), and methyl isobutyl ketone (MIBK), and (b) the low molecularweight hydrocarbons such as ethane, propane, butane, and propylene, aswell as mixtures of the foregoing ketones and mixtures of the aforesaidketones with aromatic compounds such as benzene and toluene. Inaddition, halogenated low molecular weight hydrocarbons, such as C -Cchlorinated hydrocarbons, e.g., dichloromethane and dichloroethane andmixtures thereof, may be used as solvents. Specific examples of suitablesolvent mixtures are methyl ethyl keytone and methyl isobutyl ketone,methyl ethyl ketone and toluene, dichloromethane and dichloroethane, andpropylene and acetone.

The preferred solvents are ketones with methyl ethyl ketone beingparticularly preferred. A particularly preferred solvent mixture is amixture of methyl ethyl ketone and methyl isobutyl ketone.

During the operation of the process of the present invention, thepetroleum oil stock is fed to the cooling tower 3 at a temperature aboveits pour and cloud point. In the case of an oil fraction containing arelatively small amount of wax, the oil may be fed at ambienttemperature. In the case of an oil containing a relahowever, it will beapparent to those skilled in the art that higher or lower feed rates canbe used.

The solvent, orsolvent mixture, will be prechilled to a temperaturesufficient to permit cooling of the oil to the desiredv temperature. Theexact temperature employed will depend upon the amount of oil to becooled and the amount of solvent to be added to the oil; i.e., thedegree of dilution which is sought during the filtration step. Theprechilled solvent is added incrementally along the length of thecooling tower so as to maintain an average chilling rate at or belowabout 8F./minute and preferably between about 1 to about F./minute.

tively large amount of wax an elevated temperature will be used. Ingeneral, the wax content of the oil feed will vary between ten andtwenty-five weight percent and the pour and cloud points will range,respectively, between and l70F. and and 175F. In general, a feed ratebetween and 600 bbl./hour will be used;

In general, the amount of solvent added will be sufficient to provide aliquid/solid weight ratio between the range of 5/ l and 20/1 at thedewaxing temperature and a solvent/oil volume ratio between 1.5/1 and5/1.

An essential feature of this invention is the maintenance of a highdegree of agitation in at least a portion of the stages during chilling.In general, the degree of agitation must be sufficient to providesubstantially instantaneous mixing; i.e., substantially complete mixingof the oil-solvent mixture in one second or less. In this way, thedeleterious effects of shock chilling are offset, the chilling rate ismore readily controlled and increased filtration rates are obtained. Thedegree of agitation required in this invention can be achieved by increasing the agitator RPM when all other mixing variables; e.g., flowrate through the mixer, vessel and agitator design, viscosity of theingredients, etc. are maintained constant. In general, the degree ofagitation required in this invention can be achieved when the modifiedReynolds Number, (Perry, Chemical Engineer's Handbook, 3rd, p. 1224,McGraw-l-lill, New York, 1959), N e, which is defined by the equation:

where L agitator diameter, ft.

l= liquid density, lb./ft.

n agitator speed, r.p.s.

,u.= liquid viscosity, lb./ft. sec. is between about 200 and about150,000.

The dimensionless ratio of cooling tower diameter to agitator diameteris between about l.5/l and about 10/1, and the ratio of the impellerblade length to impeller blade width ranges from about 0.75 to 2 andpreferably from about 1 to 1.5. The ratio of the mixing stage height tothe diameter of the stage will generally range from about 0.2/1-1/1. Aturbine type agitator is preferred; however, other types of agitatorssuch as propeller agitators may be used.

The cooling tower may or may not be baffled, but 5" baffled tower ispreferred. Each stage will generally contain from about 2-8 bafiles andpreferably from 2-4 baffles located about the outer periphery of eachstage. The width of the baffles may range from about 5-15 percent of thediameter of the tower. In general, the dimensionless ratio of thecross-section of the restricted flow opening to the cross-section of thetower will be between about 1/20 and about 1/200.

In general, the cooling tower of the present invention will be operatedat a pressure sufficient to prevent flashing of the solvent. Atmosphericpressure is sufficient when the ketones are employed as solvents;however, superatmospheric pressures are required when low molecularweighthydrocarbons, such as propane, are used.

PREFERRED EMBODIMENT The invention will be more apparent from thepreferred embodiment and working examples set forth below. Turning againto the drawing, a lubricating oil distillate fraction, having a boilingrange from about 850 to 950F., a wax content of from about 18 to about21 wt. percent, and initial pour and cloud points between about 1 l and120F. and 1 12 and 125F., respectively, is fed from the storage tank 1through line 2 to the first stage 4(a) of the cooling tower 3. A solventsuch as methyl ethyl ketone or a mixture of 55 wt. percent methylisobutyl ketone and 45 wt. percent methyl ethyl ketone is passed fromstorage tank 5 through line 6 to the heat exchangers 7 and 8, where thesolvent is cooled by conventional means to a temperature between l5 and25F.

In the preferred embodiment cold filtrate from the separator 21 is usedto precool the solvent in heat exchanger 7. The filtrate enters throughline 24 and leaves through line 26. The solvent is further cooled inheat exchanger 8 with liquid propane or other suitable coolant, enteringthrough line 25 and leaving through line 27. It is preferred that thesolvent be prechilled to a temperature which will permit cooling of theoil to a temperature between about 20F. and 25" F. although lowertemperatures can be used.

The cooled solvent passes from the heat exchanger 8 through line 9 tothe manifold 10. in the preferred embodiment the cooling tower isdivided into 16 stages, 4, and the manifold consists of 16 parallelsolvent inlets 12. The flow rates are adjusted to give the desiredtemperature drop per stage. Each of the 16 separate stages is providedwith an agitator turbine 12 which is turned at a sufficiently high RPMto produce adequate mixing. The oil and solvent entering the first stage4(a) is substantially instantaneously mixed. As the oil-solvent mixturepasses downwardly through the cooling tower, it is substantiallyinstantaneously mixed with additional solvent which is added to eachstage. As a result of the instantaneous mixing of the warmer oil or oilsolvent mixture with the added solvent, the temperature of the totalmixture is substantially instantaneously reduced in each stage. In thisway, the adverse effects of shock cooling are minimized.

The oil-solvent mixture leaves the bottom of the cooling tower as amixture containing precipitated wax at a temperature ranging from about0-50F. The solution-wax mixture passes through line 14 to a separatingmeans 15 which is preferably a rotary vacuum filter. The wax-solvent ispassed through line 16 to a liquidliquid separating means 19 which ispreferably distillation. The wax is recovered through line 18 and theketone solvent is recovered through line 17. The oilsolvent solution ispassed through line 20 to a liquidliquid separating means which ispreferably distillation. The dewaxed oil is recovered through line 22and the mixed ketones solvent is recovered through line 23. The dewaxedoil which now has a reduced pour point and cloud point may be useddirectly or subjected to further dewaxing by conventional means such asthe well known ketone or propane dewaxing methods.

EXAMPLE 1 Using the preferred embodiment described above, a lubricatingoil having the following specifications:

Boiling Range 800 to 950F.

Gravity, API 302 7 Dry Wax Content 20 Wt. percent at 30F. Pour Point F.Cloud Point 118F. was dewaxed. The oil fraction was fed at a rate of200.

cc./min. and at a temperature of F. The ketone mixture'described abovewas used as the solvent. The

TABLE I Steady state temperature, F. of each stage Stage l 2 3 4 5 6 7 8Temp 123. 2 116.4 109.6 102.8 96.0 89.2 82.4 75.6

Stage 9 10 11 12 13 14 15 16 Temp 68.8 62.0 55.2 48.4 41.6 34.8 28.021.2

The average chilling rate was 2F./min.

The precipitated wax was removed from the oilsolvent solution byfiltration. The oil was recovered from solution by distillation. Therecovered oil had the following specifications:

Boiling Range 800 to 950F.

Gravity, API 29.0

Dry Wax Content 0 Wt. percent at 30F.

Pour Point 25-F.

Cloud Point 31F.

The cooling tower employed in this and the following examples was fourfeet in height and one-half foot in diameter. The solvent inlets werepositioned uniformly along the height of the tower at one-half footintervals. The agitator consisted of 16 impellers positioned along theheight of a single shaft. The impeller diameter was 2 inches.

EXAMPLES 2-4 1 To illustrate the effect of the degree of agitation uponboth the filter rate and extent of separation between the solvent andwax, the following runs were made in the manner disclosed above usingthe cooling tower, oil fraction and solvent of Example 1. The oil andsolvent flow rates were adjusted so as to maintain an average chill rateof about 2F./min. The mixed solvent was prechilled to 20F. The agitatorRPM was set at 200, 400 and 600 corresponding to modified ReynoldsNumbers of 5.55K*, 11.1 1K* and 16.67K, respectively. The resultsobtained are shown in Table II, below.

both I and p. have different values in each stage due to the effect ofdilution chilling and precipitation.

It will be apparent from the foregoing results that both the filtrationrate and oil yield increase with increasing agitation as reflected bythe Reynolds Numbers.

EXAMPLES -8 To show the effect of chilling rate upon the filtrationrate, the following runs were completed using the method described inExample 1. The solvent and feed.

rates were varied to vary the chilling rate. The agitator speed wasconstant in all runs at 600 RPM. The results are shown in Table III.below.

TABLE III Effect of Chilling Rate on Filtration Rate Ex. 5 Ex. 6 Ex. 7Ex. 8 Average Chilling Rate, F./min. 0.5 l 2 3 Filtration Rate U.S.G.Dewaxed Oil Per FLIHI'. 4.8 4.6 4.0 3.4

It will be apparent from the foregoing results that the filtration rate,over the range shown, decreases as the chilling rate increases.

The above process has been described in connection with the separationof wax fromoil. However, it may also be used for crystallization andpurification of other materials. For example, it can be used torecrystallize slack waxes or petrolatums or with any feed in which thereis a crystallizable material which can be separated from the feed oncooling. Thus, it can be used for paraxyle'ne separation, fatty acidpurification, dichlorobenzene isomer separation and the like.Additionally it can be used for solvent purification such as benzene,pharmaceutical purification and the like.

What is claimed is:

l. A process for dewaxing a waxy petroleum oil stock comprisingintroducing said waxy oil stock into a chilling zone divided into aplurality of stages, introducing a cold dewaxing solvent, into at leasta portion of said stages, maintaining a high degree of agitation in aplurality of the solvent-containing stages 56' as to effect asubstantially instantaneous mixing of said solvent and 7 is divided intoat least 6 agitated stages.

3. The process of claim 1 wherein the cold dewaxing solvent is a mixtureof propylene and acetone.

4. The process of claim 1 wherein the cold dewaxing solvent is chosenfrom the group consisting of ketones having 3 to 6 carbon atoms andtheir mixtures.

5. The process of claim 1 wherein the cold dewaxing solvent is selectedfrom the group consisting of low molecular weight hydrocarbons and theirmixtures.

6. The process of claim 1 wherein the cold dewaxing solvent is selectedfrom the group consisting of halogenated hydrocarbons.

7. The process of claim 1 wherein the degree of agitation in each ofsaid stages is maintained between about 200 and 150,000 modifiedReynolds Number.

8. The process of claim 1 wherein said solvent-oil mixture is cooled insaid chilling zone at a rate of about l-5F. per minute.

9. The process of claim 1 wherein the petroleum oil stock is alubricating oil fraction.

10. A process for dewaxing a waxy petroleum oil stock comprisingintroducing said oil stock into a chilling zone divided into at least 6agitated stages, introducing a cold dewaxing solvent into each of saidagitated stages, maintaining a high degree of agitation in each of saidstages so as to effect substantially instantaneous mixing of saidsolvent and waxy oil in each of said agitated stages while cooling thesolvent-waxy oil mixture at an average rate of about l8F. per minute asit progresses through the chilling zone thereby precipitating asubstantial portion of said wax from said mixture under conditions ofsaid high degree of agitation and effecting a final temperature of saidmixture in said chilling zone of between about 0 and 50F., withdrawingthe solvent-oil mixture containing precipitated wax from said chillingzone, separating the precipitated wax from the oil-solvent mixture andrecovering the petroleum oil stock having a reduced wax content fromsaid mixture.

11. A process for dewaxing a waxy petroleum oil stock comprisingintroducing said oil stock into a chilling zone divided into at least 6agitated stages, introducing a cold dewaxing solvent into each of saidagitated stages, maintaining the degree of agitation in each of saidstages between about 200 and 150,000 modified Reynolds Number so as toeffect substantially instantaneous mixing of said solvent and oil ineach of said agitated stages while cooling said mixture at an averagerate of about l8F. per minute as it progresses through the chilling zonethereby precipitating a substantial portion of said wax from saidmixture under conditions of said high degree of agitation and effectinga final temperature of said mixture in said chilling zone of betweenabout 0 and 50F, withdrawing the oilsolvent mixture containingprecipitated wax from said chilling zone, separating the precipitatedwax from the oil-solvent mixture and recovering the petroleum oil stockhaving a reduced wax content from said mixture.

12. The process of claim 11 wherein the cold dewaxing solvent ispropylene-acetone.

13. A process for dewaxing a waxy lubricating oil stock comprisingintroducing said petroleum oil stock into the top of a chilling zonedivided into between about 6 and 50 agitated stages, introducing a colddewaxing solvent into each of said stages, maintaining the degree ofagitation in each of said stages between about 200 and 150,000 modifiedReynolds Number so as to effect substantially instantaneous mixing ofsaid solvent and said oil in each of said agitated stages while coolingsaid mixture at an average rate of about 1-5F. per minute as itprogresses down the chilling zone thereby precipitating a substantialportion of said wax from said mixture under conditions of said highdegree of agitation and effecting a temperature of said mixture in thefinal agitated stage of between about 0 and 40F, withdrawing theoil-solvent mixture containing precipitated wax from said chilling zone,separating the precipitated wax from the oil-solvent mixture andrecovering a pe: troleum oil stock having a reduced wax content fromsaid mixture.

14. A process for separating a wax-oil mixture comprising introducingsaid mixture, at a temperature above its cloud point, into a chillingzone divided into a plurality of stages, introducing a dewaxing solventinto at least a portion of said stages, maintaining a high degree ofagitation in a plurality of the solventcontaining stages so as to effecta substantially instantaneous mixing of said solvent and said wax-oilmixture while cooling said mixture as it progresses through saidchilling zone thereby precipitating a substantial portion of said waxfrom said mixture under conditions of said high degree of agitation.

1 5. A process for dewaxing a waxy petroleum oil stock comprisingintroducing said petroleum oil stock into a chilling zone divided into aplurality of stages, introducing a dewaxing solvent into a plurality ofsaid stages, maintaining a high degree of agitation in each of saidstages so as to effect a substantially instantaneous mixing of saidsolvent and said oil while cooling said solvent-oil mixture as itprogresses through said chilling zone thereby precipitating asubstantial portion of the wax from said mixture under conditions ofsaid high degree of agitation, separating precipitated wax from saidsolvent-oil mixture and recovering a petroleum oil stock having areduced wax content from said mixture.

16. A process for dewaxing a waxy petroleum oil stock comprisingintroducing said petroleum oil stock into a chilling zone divided into aplurality of stages, in-

troducing a dewaxing solvent into a plurality of said stages,maintaining ahigh degree of agitation in each of said stages so as toeffect a substantially instanta-- neous mixing of said solvent and saidoil while cooling said solvent-oil mixture at an average rate of about-8F. per minute as it progresses through said chilling zone therebyprecipitating a substantial portion of the wax from said mixture underconditions of said high degree of agitation, separating precipitated waxfrom the oil-solvent mixture and recovering a petroleum oil stock havinga reduced wax content from said mixture.

17. The process of claim 1 wherein said dewaxing solvent comprises amixture of methylethyl ketone and toluene.

18. The process of claim 10 wherein said dewaxing solvent comprises amixture of methylethyl ketone and toluene.

19. The process of claim 11 wherein said dewaxing solvent comprises amixture of methylethyl ketone and toluene.

toluene.

2. The process of claim 1 wherein the chilling zone is divided into atleast 6 agitated stages.
 3. The process of claim 1 wherein the colddewaxing sOlvent is a mixture of propylene and acetone.
 4. The processof claim 1 wherein the cold dewaxing solvent is chosen from the groupconsisting of ketones having 3 to 6 carbon atoms and their mixtures. 5.The process of claim 1 wherein the cold dewaxing solvent is selectedfrom the group consisting of low molecular weight hydrocarbons and theirmixtures.
 6. The process of claim 1 wherein the cold dewaxing solvent isselected from the group consisting of halogenated hydrocarbons.
 7. Theprocess of claim 1 wherein the degree of agitation in each of saidstages is maintained between about 200 and 150,000 modified ReynoldsNumber.
 8. The process of claim 1 wherein said solvent-oil mixture iscooled in said chilling zone at a rate of about 1*-5*F. per minute. 9.The process of claim 1 wherein the petroleum oil stock is a lubricatingoil fraction.
 10. A process for dewaxing a waxy petroleum oil stockcomprising introducing said oil stock into a chilling zone divided intoat least 6 agitated stages, introducing a cold dewaxing solvent intoeach of said agitated stages, maintaining a high degree of agitation ineach of said stages so as to effect substantially instantaneous mixingof said solvent and waxy oil in each of said agitated stages whilecooling the solvent-waxy oil mixture at an average rate of about 1*-8*F.per minute as it progresses through the chilling zone therebyprecipitating a substantial portion of said wax from said mixture underconditions of said high degree of agitation and effecting a finaltemperature of said mixture in said chilling zone of between about 0*and 50*F., withdrawing the solvent-oil mixture containing precipitatedwax from said chilling zone, separating the precipitated wax from theoil-solvent mixture and recovering the petroleum oil stock having areduced wax content from said mixture.
 11. A process for dewaxing a waxypetroleum oil stock comprising introducing said oil stock into achilling zone divided into at least 6 agitated stages, introducing acold dewaxing solvent into each of said agitated stages, maintaining thedegree of agitation in each of said stages between about 200 and 150,000modified Reynolds Number so as to effect substantially instantaneousmixing of said solvent and oil in each of said agitated stages whilecooling said mixture at an average rate of about 1*-8*F. per minute asit progresses through the chilling zone thereby precipitating asubstantial portion of said wax from said mixture under conditions ofsaid high degree of agitation and effecting a final temperature of saidmixture in said chilling zone of between about 0* and 50*F., withdrawingthe oil-solvent mixture containing precipitated wax from said chillingzone, separating the precipitated wax from the oil-solvent mixture andrecovering the petroleum oil stock having a reduced wax content fromsaid mixture.
 12. The process of claim 11 wherein the cold dewaxingsolvent is propylene-acetone.
 13. A process for dewaxing a waxylubricating oil stock comprising introducing said petroleum oil stockinto the top of a chilling zone divided into between about 6 and 50agitated stages, introducing a cold dewaxing solvent into each of saidstages, maintaining the degree of agitation in each of said stagesbetween about 200 and 150,000 modified Reynolds Number so as to effectsubstantially instantaneous mixing of said solvent and said oil in eachof said agitated stages while cooling said mixture at an average rate ofabout 1*-5*F. per minute as it progresses down the chilling zone therebyprecipitating a substantial portion of said wax from said mixture underconditions of said high degree of agitation and effecting a temperatureof said mixture in the final agitated stage of between about 0 and40*F., withdrawing the oil-solvent mixture cOntaining precipitated waxfrom said chilling zone, separating the precipitated wax from theoil-solvent mixture and recovering a petroleum oil stock having areduced wax content from said mixture.
 14. A process for separating awax-oil mixture comprising introducing said mixture, at a temperatureabove its cloud point, into a chilling zone divided into a plurality ofstages, introducing a dewaxing solvent into at least a portion of saidstages, maintaining a high degree of agitation in a plurality of thesolvent-containing stages so as to effect a substantially instantaneousmixing of said solvent and said wax-oil mixture while cooling saidmixture as it progresses through said chilling zone therebyprecipitating a substantial portion of said wax from said mixture underconditions of said high degree of agitation.
 15. A process for dewaxinga waxy petroleum oil stock comprising introducing said petroleum oilstock into a chilling zone divided into a plurality of stages,introducing a dewaxing solvent into a plurality of said stages,maintaining a high degree of agitation in each of said stages so as toeffect a substantially instantaneous mixing of said solvent and said oilwhile cooling said solvent-oil mixture as it progresses through saidchilling zone thereby precipitating a substantial portion of the waxfrom said mixture under conditions of said high degree of agitation,separating precipitated wax from said solvent-oil mixture and recoveringa petroleum oil stock having a reduced wax content from said mixture.16. A process for dewaxing a waxy petroleum oil stock comprisingintroducing said petroleum oil stock into a chilling zone divided into aplurality of stages, introducing a dewaxing solvent into a plurality ofsaid stages, maintaining a high degree of agitation in each of saidstages so as to effect a substantially instantaneous mixing of saidsolvent and said oil while cooling said solvent-oil mixture at anaverage rate of about 1*-8*F. per minute as it progresses through saidchilling zone thereby precipitating a substantial portion of the waxfrom said mixture under conditions of said high degree of agitation,separating precipitated wax from the oil-solvent mixture and recoveringa petroleum oil stock having a reduced wax content from said mixture.17. The process of claim 1 wherein said dewaxing solvent comprises amixture of methylethyl ketone and toluene.
 18. The process of claim 10wherein said dewaxing solvent comprises a mixture of methylethyl ketoneand toluene.
 19. The process of claim 11 wherein said dewaxing solventcomprises a mixture of methylethyl ketone and toluene.
 20. The processof claim 13 wherein said dewaxing solvent comprises a mixture ofmethylethyl ketone and toluene.
 21. The process of claim 14 wherein saiddewaxing solvent comprises a mixture of methylethyl ketone and toluene.22. The process of claim 15 wherein said dewaxing solvent comprises amixture of methylethyl ketone and toluene.
 23. The process of claim 16wherein said dewaxing solvent comprises a mixture of methylethyl ketoneand toluene.